[单选题]
A boat can travel 4.0 m/s in stilngnh v ra2 /vqp0h-45al w-a,t -xwlu-bfq29ly z. tucv) ater. With what speed, relative to the shore, does it move in a river that is flowing at 1.0 m/s if the boat is heading dowlu -y vqcwl9 -2)uz.fxbta t-,nstream?
